Editing IPCC:AR6/WGI/Chapter-12 (section)

=== 12.3.6 Oceanic ===

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Oceanic changes and impacts were a substantial focus of SROCC ( [[#IPCC--2019b|IPCC, 2019b]] ). [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] of this Report assesses changes in ocean processes, and here we note major connections used by scientists to understand how oceanic CIDs affect ecosystems and society.

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==== 12.3.6.1 Mean Ocean Temperature ====

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Shifts in thermal zones affect the suitability of fisheries and marine and coastal species habitat and migration routes ( [[#Hoegh-Guldberg--2010|Hoegh-Guldberg and Bruno, 2010]] ; [[#Doney--2012|Doney et al., 2012]] ; [[#Burrows--2014|Burrows et al., 2014]] ; [[#Urban--2015|Urban, 2015]] ; [[#Hixson--2016|Hixson and Arts, 2016]] ; [[#Tripathi--2016|Tripathi et al., 2016]] ; N. [[#Ahmed--2019|]] [[#Ahmed--2019|Ahmed et al., 2019]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ). Intertidal species are particularly dependent on suitable conditions for both air and sea surface temperatures ( [[#Monaco--2019|Monaco and McQuaid, 2019]] ). The structure of ocean warming also affects the intensity of upper-ocean stratification and the timing and strength of coastal upwelling (driven also by mean wind changes), which alters the vertical transport of oxygen- and nutrient-rich waters affecting fishery and marine ecosystem productivity (D. [[#Wang--2015|]] [[#Wang--2015|Wang et al., 2015]] ).

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==== 12.3.6.2 Marine Heatwave ====

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Marine heatwaves (MHW) push water temperatures above key thresholds and have been associated with coral bleaching episodes, species shifts and harmful algal blooms that can disrupt ecosystems, tourism and human health (Box 9.2; [[#Wernberg--2016|Wernberg et al., 2016]] ; [[#Arias-Ortiz--2018|Arias-Ortiz et al., 2018]] ; [[#Oliver--2018|Oliver et al., 2018]] ; [[#Frölicher--2019|Frölicher, 2019]] ; [[#Smale--2019|Smale et al., 2019]] ; [[#Sully--2019|Sully et al., 2019]] ). The duration and return period of marine heatwaves provide insight into aggregate stresses on marine species, fisheries and ecosystems, with various indices gauging cumulative intensity or the number of days, weeks or months exceeding critical thresholds ( [[#Frieler--2013|Frieler et al., 2013]] ; [[#Frölicher--2018|Frölicher et al., 2018]] ; [[#Hughes--2018b|Hughes et al., 2018b]] ; [[#Cheung--2020|Cheung and Frölicher, 2020]] ). [[#Hobday--2016|Hobday et al. (2016)]] defined marine heatwaves as the exceedance of the 90th percentile of the sea surface temperature (SST) distribution on a given Julian day during five or more consecutive days, while Box 9.2, Figure 1 shows MHW as an exceedance of 99th-percentile 11-day de-seasonalized SSTs. The return period of marine heatwaves is also critical in determining a coral system’s ability to recover before the next event ( [[#Hughes--2018a|Hughes et al., 2018a]] ).

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==== 12.3.6.3 Ocean Acidity ====

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Uptake of atmospheric CO <sub>2</sub> and subsequent increases in dissolved CO <sub>2</sub> lowers ocean pH and can reduce carbonate ion concentrations below critical calcium carbonate saturation thresholds for marine and aquatic organisms growth, reproduction and/or survival, with extended implications for marine ecosystems including fisheries ( [[#Bell--2013|Bell et al., 2013]] ; [[#Kroeker--2013|Kroeker et al., 2013]] ; [[#Barange--2014|Barange et al., 2014]] ; [[#Dutkiewicz--2015|Dutkiewicz et al., 2015]] ; [[#Ekstrom--2015|Ekstrom et al., 2015]] ; [[#Gattuso--2015|Gattuso et al., 2015]] ; [[#Mathis--2015a|Mathis et al., 2015a]] ; [[#Nagelkerken--2015|Nagelkerken and Connell, 2015]] ; [[#Behrenfeld--2016|Behrenfeld et al., 2016]] ; [[#Nagelkerken--2016|Nagelkerken and Munday, 2016]] ; [[#Tripathi--2016|Tripathi et al., 2016]] ; [[#Jiang--2018|Jiang et al., 2018]] ; [[#Weiss--2018|Weiss et al., 2018]] ; N. [[#Ahmed--2019|]] [[#Ahmed--2019|Ahmed et al., 2019]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ). Lower pH may provide more favourable conditions for toxic algal blooms ( [[#Riebesell--2018|Riebesell et al., 2018]] ) and can interact with hypoxic zones to impact ecosystems ( [[#Gobler--2016|Gobler and Baumann, 2016]] ; [[#Cai--2017|Cai et al., 2017]] ).

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==== 12.3.6.4 Ocean Salinity ====

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Changes in currents, sea ice brine rejection and net freshwater flux in the ocean can alter salinity with effects on mixed layer structure, density stratification and the vertical movement of nutrients and marine organisms ( [[#Freeland--2013|Freeland, 2013]] ; [[#Haumann--2016|Haumann et al., 2016]] ).

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==== 12.3.6.5 Dissolved Oxygen ====

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Ocean warming and increased stratification decrease the oxygen content of the ocean ( [[#Griffiths--2017|Griffiths et al., 2017]] ; [[#Schmidtko--2017|Schmidtko et al., 2017]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ), lead to an expansion of oxygen minimum zones in the open ocean ( [[#Stramma--2012|Stramma et al., 2012]] ; [[#Zhang--2013|Zhang et al., 2013]] ) and exacerbate the creation of anoxic ‘dead zones’ in the coastal oceans ( [[#Breitburg--2018|Breitburg et al., 2018]] ). Such a decline (characterized by successive dissolved oxygen concentration thresholds) could affect a wide range of marine organisms and reduce marine habitats ( [[#Chan--2008|Chan et al., 2008]] ; [[#Vaquer-Sunyer--2008|Vaquer-Sunyer and Duarte, 2008]] ; [[#Hoegh-Guldberg--2010|Hoegh-Guldberg and Bruno, 2010]] ; [[#Altieri--2015|Altieri and Gedan, 2015]] ; [[#Breitburg--2018|Breitburg et al., 2018]] ) and can also lead to further local acidification ( [[#Zhang--2016|Zhang and Gao, 2016]] ; [[#Laurent--2017|Laurent et al., 2017]] ).

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